(1) Field of the Invention
The present invention relates to the removal of sulfur oxides from exhaust gases, and more particularly to the removal of sulfur oxides from exhaust gases while avoiding fume formation and recovering a valuable product derived from the sulfur oxides.
(2) Description of the Related Art
Sulfur oxides (generically identified as So.sub.x) are a common component of exhaust gases generated by boilers in steam and electric power production as well as by evaporators and dryers in paper pulp manufacturing plants and other industrial operations. Commonly, the sulfur oxide is sulfur dioxide, although other sulfur oxides such as sulfur monoxide (SO) or sulfur trioxide (SO.sub.3) may be present. Such sulfur oxides are known to be harmful to the environment due to their propensity to form acid rain and to be corrosive when condensed from contaminated air onto any corrodible surface.
A number of processes have been developed to minimize the evolution of sulfur oxides in exhaust gases. These can be divided roughly into those processes that remove sulfur from the fuel prior to combustion and those processes that remove sulfur oxides from exhaust gases after combustion. The latter type of processes are termed flue gas desulfurization (FGD) processes. Of the FGD processes that have reached commercial importance, those that use finely milled limestone slurries to absorb and to react with the SOX and those that use magnesium oxide in similar systems account for about 90% of current commercial FGD systems for power generation plants (the most important of the fixed source generators of SO.sub.x). Such processes commonly pump water slurries of finely milled limestone, or magnesium oxide, into a spray tower where droplets of the slurry contact the SO.sub.x -containing exhaust gas as it flows up the tower. See, e.g., U.S. Pat. No. 5,630,991. The SO.sub.x is absorbed into the water and reacted with the mineral oxides to form calcium sulfate (gypsum) or magnesium sulfate, respectively. While limestone is a relatively inexpensive raw material, these processes have disadvantages such as the disposal of the large volumes of gypsum that are produced, the high cost of milling the limestone into particles that are small enough for reasonably fast reaction, the design of process flow equipment to handle abrasive slurries of solids rather than clear solutions and the reqirement for large amounts of limestone per unit of SO.sub.x removed due to the low reactivity of the limestone.
Dry sorbent injection systems comprise most of the rest of the commercial FGD systems now used on power plants, but those systems also have disadvantages of the use of speciallized solid/liquid contacting equipment and the costs associated with moving large volumes of dry reactants and products.
Because of the dominance of the limestone scrubber process in present commercial use, many plants have existing limestone scrubbing equipment. It would be most economical, therefore, that any process that is used to replace a limestone scrubbing system be able to utilize much, if not all, of the same equipment. Thus, an alternative process for which a flue gas desulfurization system, such as a limestone scrubbing system, could easily be retrofitted would have an advantage over other alternative processes.
While liquid-based, non-slurry FGD systems have shown promise, such systems have yet to capture an appreciable part of commercial FGD applications. One of the most studied and best known of the liquid-based FGD processes is the absorption of SO.sub.x into water solutions of ammonium and ammonium salts. Such ammoniacal liquid scrubbing of exhaust gases was reported in U.S. Pat. No. 2,142,987, issued in 1939, where it was disclosed that aqueous solutions of weak acids capable of reacting with the sulfurous acid produced during the absorption of SO.sub.x in water could be used to recover SO.sub.x. After the sulfur oxide was reacted with the weak acid to produce a sulfite, the solution containing the weak acid sulfite was heated to free the sulfur oxide and regenerate the weak acid. Ammonium salts of such weak acids were said to be among those that were preferred.
Later, Wiewiorowski and Vincent, U.S. Pat. No. 3,633,339, found that aqueous ammonium phosphate solutions could be used to strip sulfur dioxide from gas streams and further disclosed the removal of the sulfur dioxide from the ammonium phosphate stream by extraction into a liquid amine. The sulfur dioxide could then be released as a pure gas by heating the amine solution. Pure gaseous sulfur dioxide was the product of the process and both the ammonium phosphate solution and the amine solution were regenerated and recycled in the process. While this process produced gaseous sulfur dioxide as a product, it required a liquid/liquid extraction operation, which have been known to be difficult to control in systems where particulate contaminants can accumulate, and also required an energy-consuming stripping step.
Purified sulfur dioxide was also recovered by Jordan et al., U.S. Pat. No. 3,927,178, who reported absorption of sulfur dioxide into a concentrated solution of ammonium sulfate into which ammonia was added in the last stage of absorption. The purpose of the process was to minimize energy cost associated with the separation of solid ammonium sulfate. The scrubber solution was mixed with molten ammonium bisulfate to release pure gaseous sulfur dioxide, which was recovered, and to form ammonium sulfate. Ammonium sulfate crystals were heated to about 700.degree. F. to free ammonia and to form molten ammonium bisulfate, both of which were recycled into the process at the points where those respective materials were used as stated above. As in the Wiewiorowski et al. process, the Jordan et al. process required heating to release gaseous sulfur dioxide, but, in addition, required a high temperature generator to melt ammonium sulfate. This increases not only the complexity of controlling the process, but also the danger associated with handling such hot material.
Witte et al., U.S. Pat. No. 3,969,492, disclosed a process that resulted in the recovery of elemental sulfur from sulfur dioxide in waste gas streams. The sulfur dioxide was absorbed into a solution of mono- and diammonium phosphate to form ammonium bisulfite. The ammonium bisulfite was then reacted with hydrogen sulfide, or other suitable reducing agents, to give a direct reduction of the sulfites to elemental sulfur and to regenerate the monoammonium phosphate to diammonium phosphate. It was also reported that absorbed sulfur dioxide could be thermally stripped from the mixed phosphate solution prior to reaction with H.sub.2 S by thermal stripping, but it was said to be desirable that such desorption be carried out in the presence of a minimal amount of oxygen to prevent the oxidation of sulfites to sulfate. This process again showed that while gaseous sulfur dioxide could be produced, energy-demanding thermal stripping was necessary and toxic hydrogen sulfide gas was required.
Haese, in U.S. Pat. No. 4,268,489, reported the absorption of sulfur dioxide by an aqueous solution of ammonium sulfite and bisulfite. A portion of the solution discharged from the scrubber is oxidized to form ammonium sulfate and that oxidized ammonium sulfate solution is then used to further treat the scrubbed gas. It was reported that the benefit of such a process was that since the ammonium sulfate had practically no vapor pressure, the second scrubbing resulted in scavenging ammonia from the gas phase and returning it to the absorption system. Ammonium sulfate could also be produced as a product.
Recently, Gal, U.S. Pat. No. 5,624,649, disclosed a process wherein ammonia/ammonium sulfate in aqueous solution was used to strip sulfur dioxide from flue gas in the presence of oxidation air. The ammonium sulfate that was produced was further contacted with ammonia and the resulting ammonia/ammonium sulfate solution was contacted with potassium chloride to form crystalline potassium sulfate, which was withdrawn from the process as a product. Ammonia was regenerated from the ammonium chloride solution exiting the crystallizer by contact with lime. Calcium chloride was removed as a byproduct. While it was reported that the potassium sulfate was a useful fertilizer product, no uses were reported for the calcium chloride that was produced.
Early in the development of ammoniacal liquid-based FGD processes it was recognized that one particular problem that was unique to these processes was the formation of a persistent plume from the stacks of plants using ammoniacal FGD scrubber systems. While visible plumes are often observed from exhaust stacks of wet scrubbing systems, such plumes usually dissipate after a short period unless the waste gas also contains fine ammonium sulfate/sulfite salt particles that are termed "fume". Such fumes are discussed by Bai et al., Ind. Eng. Chem. Res., 31:88-94, 1992; Gautney et al., J. Chem. Eng. Data, 25:154-158, 1980; and Moore, Fume formation in ammonia scrubbers, Manuscript No. 77-WA/APC-2, presented at Winter Annual Meeting of The American Society of Mechanical Engineers, Atlanta, Ga., Nov. 27-Dec. 2, 1977). Thus, while the ammoniacal FGD processes promised effective sulfur dioxide removal and reasonable costs, they suffered from the problem of forming unsightly and unacceptable visible and persistent plumes from the exhaust stacks.
One potential solution to this problem was proposed by Spector et al. in U.S. Pat. No. 3,843,789. This reference recognized that the plume or fume was generated within the absorber and was apparently caused by formation of very small particulates of ammonium sulfites in the gas phase. The solution that was proposed was to control the operating conditions in the scrubber, namely temperature and vapor pressure of sulfur oxides, ammonia and water, so that the concentrations of those three components in the gas phase were held below the level where fume formation was initiated. However, the methods disclosed for how such control was to be implemented focused on temperature control, control of the inlet concentrations of ammonia and sulfur dioxide and removal of solids from selected stages in a multi-stage absorber.
Later, others from the same group improved the control technique by disclosing that the water vapor pressure could be controlled by adding a non-volatile salt (such as ammonium sulfate) to selected parts of the absorber where depression of the water vapor pressure was desirable. See, for example, U.S. Pat. No. 4,151,263.
In other work, Matty et al., U.S. Pat. No. 4,004,966, reported a method for fume control for sulfur dioxide ammonia absorption systems that involved the control of pH in the scrubber to avoid the formation of ammonium hydroxide in the aqueous phase. It was reported that if pH was held to below about 5.9 in the scrubber and ammonium bisulfite were present, that ammonium hydroxide levels in the scrubbing liquid were low enought to avoid the formation of sulfite fume in the gas phase. The pH control was accomplished by distributing the addition of ammonia to a number of different points in the absorption system. While such control seemed to be effective, it required the use of pH sensors and controllers and a complex ammonia distribution and mixing system.
Thus, while ammoniacal liquid-based FGD systems promise advantages in effective removal of sulfur oxides at reasonable cost, problems still remain in how to properly control such systems to insure effective sulfur oxide removal from the flue gas while operating the system without fume formation; how to operate such a process to provide high sulfur dioxide removal efficiency; how to operate a process that provides clear solution scrubbing rather than slurry scrubbing; how to recover the removed sulfur oxides in the form of materials which have significant commercial value; and how to provide such processes without requirements for complex control systems or speciallized equipment so that they can be easily retrofitted into existing limestone scrubbing process equipment.